1. Field of the Invention
The present invention relates to a paper machine twin-wire former and a dewatering device to be used for paper layer forming therein.
2. Description of the Prior Art
In a twin-wire former as a paper layer forming device in a conventional paper machine, two wires each form a loop, stock is pinched there between, and as the stock is transferred, it is dewatered by various dewatering devices so that a fiber mat is gradually grown and a web is formed.
In FIG. 10, a construction of a typical twin-wire former is shown, and a paper layer forming device of the twin-wire former is described with reference to FIG. 10.
Stock 7 injected upwardly from a headbox 6 is pinched in a gap 5 of a wedge shape formed by two wires comprising a top wire 1 and a bottom wire 2 guided by a forming roll 4 and a breast roll 3, respectively. As the stock is transferred with the same velocity as the wires 1, 2, the gap 5 is narrowed, and as the stock if further transferred along an approximate curve R on a plurality of dewatering blades 9 arranged on a certain curvature R with intervals between one another on the side of the bottom wire 2, the stock is dewatered toward both sides by a dewatering pressure generated by the dewatering blades 9. A fiber mat is thus gradually grown and a web is formed.
Next, at a suction box 11 and a suction couch roll 12, dewatering by vacuum is performed. At the suction couch roll 12, the web 13 is transferred onto the bottom wire 2 and then is transferred to a next press part by a suction pick-up roll (not shown).
A water deflector 10 is disposed within a wire loop of the top wire 1, so that white water which accompanies the top wire 1 is discharged outside the system therefrom.
Further, in the construction of the twin-wire former shown in FIG. 10, as a countermeasure to meet various problems accompanying the dewatering being done on both sides at the same time, the employment of dewatering limiting shoes (dewatering limiting blades) of the Japanese laid-open patent application No. Hei 2(1990)-133689 as shown in FIG. 11 and a combination of dewatering limiting shoes (dewatering limiting blades) and dewatering shoes (dewatering blades) of the Japanese laid-open patent application No. Hei 4(1992)-194093 as shown in FIG. 12 have been disclosed.
FIG. 13 shows another dewatering device used for paper layer forming in the twin-wire former shown in FIG. 10. That is, FIG. 10 shows an example wherein a dewatering device in which dewatering blades 9 are disposed within one wire loop, or a loop of the bottom wire 2, is mainly used, FIG. 11 shows an example wherein a dewatering device 39 incorporating dewatering limiting blades is provided within one wire loop 32, and FIG. 12 shows an example wherein dewatering limiting blades 59 are disposed within a loop of a first wire 51 and downstream thereof both-side dewatering blades 60 are disposed within a loop of a second wire 52.
In the arrangement of the respective dewatering device shown in FIGS. 10, 11 and 12, the dewatering pressure generated between the wires is decided by the curvature R on which the dewatering blades 9, 39, 59, 60 are mainly arranged, the intervals with which the dewatering blades 9, 39, 59, 60 are disposed, the tensile force of the top wire 1, 31, 51 or of the bottom wire 2, 32, 52, and the dewatering resistance of the fiber mat layer formed between the two wire. There is no function of adjusting the dewatering pressure from the outside during operation.
So, what is shown in FIG. 13 is a dewatering device having the function of adjusting dewatering pressure. Dewatering blades 21 can adjust the pressing force given to the wires from the outside during operation and are disposed opposite to the conventional dewatering blades 9 as shown in FIG. 10 via the bottom wire 2 and the top wire 1.
In a case where the conventional shape of the dewatering blade is used in a mutually opposing dewatering device, the fiber mat layers formed between the wires receive a reaction force via the wires when the wires bend or jerk at the front edge portion of the dewatering blade. The fibers between the fiber mat layers are further moved and dispersed by the force.
But at the same time short fibers (or fine fibers) lose binding with long fibers and there is a strong possibility that the short fibers are washed off together with the water that is to be dewatered by the pressure acting on the mat layers. The yield of the short fibers on the dewatering blade side tends to become worse.
For this reason, countermeasures are taken in which the bending of the wire is made smaller or is shared by the front and rear edge portions. But in this case the fiber dispersion ability of the dewatering blades is lowered and as a result there is a disadvantage that the formation becomes worse.
Further, as the dewatering is done toward both sides, there is a large problem in that the short fibers of the central portion in the thickness direction of the paper layer move toward the outer layer portion, becoming fewer in the middle layer portion. Thus the binding between fibers becomes weaker and the strength in the thickness direction is lowered.
In order to prevent the lowering of the strength in the thickness direction, a countermeasure is taken wherein the dewatering ratio toward both sides is changed by use of the dewatering limiting blades as shown in FIGS. 11 and 12. But there is still no adjusting means to correspond to changes in paper making conditions such as changes of fiber length in the stock, paper making velocity, and alterations of grammage (basis weight).
That is, in the device shown in FIG. 11, it is possible to prevent the washing-off of short fibers, but it is difficult to enhance the dewatering pressure. For this reason there is a disadvantage in that the fiber dispersion ability within the mat layers is not enhanced. Further, in the device shown in FIG. 12, a paper in which a difference in nature between front and rear faces is small can be made by a construction in which both-side dewatering blades are disposed downstream of the dewatering limiting blades disposed immediately downstream of the stock being supplied. Reversely short fibers within the mat layers move to both sides, and there is a disadvantage in that the inner binding strength is lowered.